This project investigates the molecular pathways that govern the programmed death of T lineage cells both during development and in the mature peripheral immune system. Four areas have been addressed: 1) The role of co-stimulatory and co-receptor interactions in modulating a T cell receptor-induced death signal, 2) The involvement of Fas and TNF in mature T lymphocyte death, 3) The involvement of Fas in thymocyte deletion, and 4) Identification of downstream mediators of Fas and TNF- induced apoptosis. We compared the signaling requirements for activation to those required for T cell receptor (TCR)-driven programmed cell death (PCD). Both processes require TCR engagement and ligation of the CD4 co-receptor in the case of a T cell clone that recognizes antigen in the context of an MHC class II molecule. Stimulation of CD28 does not positively or negatively influence TCR-induced PCD, although it was required for IL-2 production. These results provide evidence that in mature T cells there exists a difference in the requirement for CD28 to achieve activation and IL-2 production compared to PCD. We have found that apoptosis in mature T cells can result from interactions between Fas (Apo-1/CD95) and Fas ligand. Defective peripheral T cell deletion due to mutations in the genes for Fas (Apo-1/CD95) and Fas ligand leads to autoimmune diseases resembling human systemic lupus erythematosus in lpr and gld mice, respectively. We have also demonstrated the occurrence of T cell receptor-induced apoptosis that is Fas-independent, functional in lpr or gld mice, and acting through tumor necrosis factor. Blockade of both tumor necrosis factor and Fas ligand, but neither alone, abrogates all T cell death. Analyses of mice with homozygous null mutations in the p55 or p75 tumor necrosis factor receptor genes show that the p75 receptor is sufficient for T cell apoptosis. These findings suggest a novel role for tumor necrosis factor and the p75 receptor in autoregulatory mature T cell apoptosis. In thymocytes, we found synergy between T cell receptor and Fas signalling for apoptosis. This suggests the possibility that Fas plays a role in tolerance induced by the deletion of autoreactive T lineage cells during development. Finally, using yeast-based interaction trap and homology screening methodologies we are studying downstream signalling molecules that play a role in Fas and TNF-mediated T cell apoptosis. We have identified two novel TNF receptor-associated factors or "TRAFS" and we are studying their role in the death of thymocytes and mature T cells.